The present invention relates generally to products of and methods for producing complex shapes of composite molded articles, including a snowboard with metal edges, by the process of co-injection molding.
None of the following is admitted to be prior art to the present invention.
Injection molded, including co-injection molded, articles could be vastly improved if there was a method for producing complex shapes of composite molded articles, in a single molding operation; thus optimizing the value of injection molding and maintaining or improving the characteristics expected of similar non-molded composite articles. For example, a problem that has long plagued the art of snowboard manufacturing has been the time and cost associated with manufacturing multiple layer laminated snowboards with metal edges to meet specific aesthetic and performance requirements. Injection molding has been sought after as a means of reducing such time and costs of manufacturing, but to date, has been unable to achieve all of the aesthetic, cost and performance attributes the public has come to expect of modern laminated snowboards. What is needed is a method for producing complex shapes of composite molded articles that meet or exceed the aesthetic and performance requirements expected of similar non-molded composite articles, preferably at reduced costs.
Injection Molding
Injection molding is where thermoplastic polymers are gravity-fed from a hopper into a barrel, melted by a reciprocating screw and/or electric heat and are propelled forward by a ram (piston, plunger) or the screw (used as a plunger) into mating steel or aluminum molds, which are cooled to below the heat-distortion temperature of the resin. The injected plastic material contracts as it cools (mold shrinkage) and shrinks. When cool enough to retain its shape, the plastic part is ejected from the mold.
Typically, good part design requires adequate taper (draft) of side walls, radii at inside corners, minimal variations in wall cross-sections, use of ribs 60% or less of outer wall thickness for stiffness, strength and minimal sink marks. Thermosetting polymers can also be injection-molded. For these materials, the barrels on the injection-molding machine are heated by hot water to a point safely below cross-linking temperature; the polymer is then propelled by ram or screw feed into heated molds. After they cure, the parts can be ejected while still hot because they have already thermally set or cross-linked.
Injection molding of polymers has revolutionized many industries. Most products today contain some form of plastic molded parts in them or consist entirely of plastic molded parts. Such products include toys, automobile parts, computer covers, phones, liquid containers and many, many other articles, too numerous to recite. However, modern molding operations have not been able to produce complex shapes of composite molded articles, in a single molding operation, that meet or exceed the aesthetic and performance requirements expected of similar non-molded composite articles. For certain products, aesthetic or performance requirements dictate that non-molded components, such as metals or ceramics, be utilized. However, incorporating polymer materials and non-molding materials in the single molding step has not been able to achieve the complex geometries required of some products, including molded snowboards with metal edges. For example, round washers have been embedded into molded parts as the base legs of appliances, computers and other similar articles to lift them off of the floor. However, such components do not require the metal and molded polymer to retain a bent, curved or complex shape whereby the shape of the article presents forces that separate the polymer and non-molded component.
Co-Injection Molding
Co-injection molding takes advantage of a characteristic of injection molding called fountain flow. That is, as the cavity is filled, the plastic at the melt front moves from the center line of the stream to the cavity walls. Because the walls are below the transition temperature (freeze temperature) of the melt, the material that touches the walls cools rapidly and freezes in place. This provides insulating layers on each wall, through which new melt makes its way to the melt front.
Sequential co-injection processes have two barrels and one nozzle in an injection molding machine. The skin polymer is injected into the mold first, then the core polymer is injected. The skin polymer is the material that is expected to be deposited on the cavity wall over the entire surface of the part. The core polymer displaces the skin polymer at the hot core, pushing it to fill the rest of the cavity. The end product is a sandwich-like structure, with the core polymer in the middle and the skin polymer on the surfaces of the part.
As two materials are processed, two hopper/screw/barrel assemblies are required for co-injection. A special co-injection nozzle allows the operator to alternate between the two materials with the speed, timing and accuracy necessary to optimize the co-injection application.
The advantages of this process are: the combination of two material properties into one part, and the maximization of the overall performance/cost ratio. One good example of co-injection is the use of polymer re-grind as the core material, while maintaining surface finish quality by using virgin polymer as the skin material. Other applications include using thermally more stable polymer as the core material to increase the thermal resistance of a part, or using a high melt-flow index polymer as the core material, to reduce the overall clamp force. Still other applications include using a core material that is lighter and exhibits more flexibility than the skin material to combine strength and flexibility to the desired part, while keeping the weight down. One combination may include a fiberglass loaded core material to provide a structural component to the part and a different skin material to maintain a smooth, consistent surface.
Numerous articles on the co-injection process have been authored. Some articles include, Co-Injection Molding ""95: Market Outlook, by Thomas W. Betts of Battenfeld of America, Inc; Molded-In Shielding Using the Coinjection Process, by Thomas W. Nash and Ralph J. McDonald, IBM Application Business Systems, Rochester, Minn. 55901; Coinjection Molding with Automotive Polyolefins, by Bruce R. Denison, D and S Plastics International; Recyclingxe2x80x94Why Not Use Co-Injection Molding?, by Joseph McRoskey of Co-Mack Technology, Inc. and Thomas W. Nash of Thomas W. Nash and Associates; and Co-Injection Molding: Current Applications, by Joseph McRoskey of Co-Mack Technology, Inc. (May 31-Jun. 2, 1998). The foregoing articles are incorporated herein by reference, including any drawings.
Injection and co-injection molding operations that incorporate additional non-molded components, such as metal edges, comprise additional technical hurdles to overcome, especially when such non-molded components are incorporated into a single molding operation step. Such hurdles include the need to adjust for shrinkage of the polymer as it cools in the mold; securing the non-molded components within the mold and eliminating or minimizing surface lines from inserting components into the mold. Previous attempts to address such technical matters have not succeeded in producing a snowboard that meet the expectations of an industry dominated by laminated snowboards.
Some molded snowboards comprise a low-grade plastic material with metal edges. Such snowboards lack smooth continuous curves and surfaces, lack a proper camber, tend to warp, and contain exposed metal points. Such irregularities may be caused by inserting plugs into holes made for binding fasteners, methods of holding the edges in place during the molding process and by the shrinkage of inconsistent material densities. Other injection molded snowboards leave space for metal edges and binding fasteners to be inserted and glued to the injection molded section of the snowboard as a secondary process. Such additional process steps are time consuming and expensive and produce a snowboard that suffers from some of the same delamination issues as traditional laminated snowboards.
Laminated Snowboards
Most modern snowboards are produced by laminating several materials together. Such snowboards are produced by companies such as Ride Snowboards, K2, Saloman, Burton and Morrow, to name a few. Examples of modern laminated snowboards can be found on web sites operated by these companies, including http://www.ridesnowboards.com, http://www.burton.com, http://www.k2snowboards.com, http://www.salomonsports.com/northamerica/snowboarding/connect/pub.html, and http://www.morrowsnowboards.com. This list is not an exhaustive list as there are hundreds of snowboard companies in business, owning to the popularity of the sport. Further examples may be found in U.S. Pat. Nos. 5,769,445; 5,782,482; 5,823,562; 5,851,331; 5,855,389; and 5,871,224.
The modern snowboard laminating technique consists of sandwiching multiple layers together with resins and industrial adhesives. Such layers typically include a wood or foam core, fiber reinforcement layer, thermoplastic layers, metal edge material, base material and a monocoque envelope. Other features may include the use of reinforcing materials in the end portions of the snowboard as an attempt to prevent delamination. Graphics may be applied below the top surface or monocoque envelope or to the top of it. In the sandwiched layers a set of screw threads or binding fasteners are inserted for securing bindings to the snowboard. Laminated boards have set the standard for the snowboard industry in terms of aesthetics and performance. The boards are strong enough to withstand the pressures applied by snowboard riders and flexible enough to absorb the shocks applied through maneuvers.
Notwithstanding the above, laminated boards experience several disadvantages. The laminated layers are held together by adhesives. The layers separate over prolonged use. Moreover, ice and snow can penetrate into the cracks between adhered layers and destroy the structural integrity of the snowboards, thereby delaminating the layers and the metal edges. The inserted screw threads for securing bindings depend on the strength of the adhesives. The screw threads may sometimes loosen and spin within the boards. As well, modern laminating techniques are time consuming, environmentally harmful and expensive to operate, requiring hours of labor for adhering layers, inserting screw threads and shaping the product.
What is needed is a method for producing complex shapes of composite molded articles, including snowboards, that meet or exceed the aesthetic and performance requirements expected of similar non-molded composite articles, preferably at reduced costs and using recyclable materials.
The present invention comprises products of and methods for producing complex shapes of composite molded articles, including snowboards, that meet or exceed the aesthetic, cost and performance requirements expected of similar non-molded composite articles. The injection molded or co-injection molded snowboard comprises a top surface and a bottom surface shaped to provide a center portion, at least one tip or tail portion and edges along the sides of the center portion, wherein the bottom surface is a substantially smooth continuous surface, the center portion is cambered away from the top surface and contains metal edges along the sides of the bottom surface center portion, the tip or tail portions are curved away from the bottom surface of the snowboard and the top surface contains binding mounts or screw threads flush mounted to secure bindings.
A preferred method of constructing a snowboard comprises co-injection molding, utilizing a skin polymer with a smooth finish for exterior portions of the snowboard and a core polymer that is lighter, structurally stronger and potentially cheaper than the skin polymer for the interior of the snowboard. A mold cavity is designed for the desired shape of the snowboard. In addition to providing for the shape of the snowboard, the mold cavity is designed to accommodate inserts for side metal edges and clips to secure such edges, if necessary, and inserts for top, flush mounted binding mounts or screw threads as well as the clips to secure such mounts. An additional set of metal components may be embedded within the top surface of the snowboard to compensate for warping away from the bottom metal edges due to the polymer shrink rate. The mold cavity must be designed to accommodate inserts for securing such metal components if such components are utilized.
As the polymer in the mold cools, it shrinks and the shrinkage rate depends on the polymer material selected. The metal edge material that is secured into the molten polymer does not shrink and therefore the shrinking polymer tends to xe2x80x9cpullxe2x80x9d the embedded metal component with it. This shrinkage and pulling action warps or bends the metal edge. The bottom surface metal edges contour to the final cure form of the majority polymer component, which for a snowboard is upward toward the top surface of the snowboard.
Several methods to adjust for the shrinkage rate of a particular polymer material may be employed. One method is to adjust the curvature of the snowboard mold cavity so that the warping will provide the desired curvature when cooling is completed. Another method is to adjust the composition of the polymer material, through additives or glass loading, to control the warping to provide the desired curvature when cooling is completed. Since the metal edges along the bottom surface of the snowboard are designed to be at a set camber after cooling, the exact adjustment necessary to achieve the desired camber after cooling involves an iterative process depending on the polymer material selection.
Another method to adjust for the shrinkage rate of a particular polymer material is to embed additional metal components similar to the bottom edge materials, into the top surface of the mold. Therefore, as the polymer material cools and shrinks, it will pull away from both the top and bottom portions in substantially equal proportions, thereby counteracting each other and producing the desired shape. Care must be taken to embed the top metal edges into the top surface so that they do not stick out of the top surface and so that they are not embedded so deep as to have no effect. The preferred depth is in the range of 0.015 to 0.060 inches (0.038 to 0.152 cm) for a snowboard that is approximately 0.5 inches (1.27 cm) thick. Proper placement involves an iterative process depending on the polymer material selection.
The mold cavity is split in two halves, an xe2x80x9cAxe2x80x9d half and a xe2x80x9cBxe2x80x9d half, with the A half representing the top of the snowboard article being manufactured and the B half representing the bottom of the snowboard article being manufactured. Loaded into each side of the B half is a B side rail insert comprising a metal rail with locator clips attached to the metal rails at about 3.5 inches (8.9 centimeters) apart center to center from adjacent clips whereby the locator clips are in molded cavities of the B side rail insert. Loaded into the A half is (i) parallel to, but not flush with, each side of the A half is an A side rail insert comprising a metal rail with locator clips attached to the metal rails at about 3.5 inches (8.9 cm) apart center to center from adjacent clips whereby the locator clips are in molded cavities of the A side rail insert and (ii) two A side nut inserts, each nut insert comprising six (6) nuts secured in place on the nut insert by six millimeter (6 mm) screws from the back of the nut insert The distance between locator clips depends on the size of the snowboard. The goal is to allow the A side metal rail to xe2x80x9cfloatxe2x80x9d a bit within the mold, so proper placement and location of clips is an iterative process depending on the size of the snowboard. The inserted nuts comprise a base that is wider than the threading such that the wider base is embedded within the molded polymer. In order to resist spinning within the molded part, as determined by screw-retention strength standards of binding mounts for alpine skis, in compliance with ASTM Standard Nos. F474-98 and F475-77 (1992), the wider base is preferably a polygon or other non-rounded shape such as a hexagon.
The locator clips for the A side rails are designed to be a specific size in order to embed the top metal rails into the top surface of the snowboard so that the metal rails do not stick out of the top surface and so that the metal rails are not embedded so deep as to have no effect on the curvature of the board as described above. Proper placement involves an iterative process depending on the polymer material selection. A depth change of just five thousandths of an inch (0.005 inches or 0.013 centimeters) causes a difference in the shape of the snowboard product.
Once all of the inserts have been located within the A and B side cavities of the mold, the mold is closed and the co-injection molding cycle is operated. The mold is then opened and the snowboard is removed from the B side of the mold. The locator clips on the B side of the mold break off, producing small plastic nubs. The screw threads holding the inserts for the binding nuts are removed. The A side locator clips are cut off and milled down.